Device for measuring the temperature of a molten metal

ABSTRACT

A device for measuring the temperature of a melt, particularly of a molten metal, comprises an optical fiber and a guiding tube having an immersion end and a second end opposite to the immersion end. The optical fiber is partially arranged in the guiding tube. An inner diameter of the guiding tube is larger than an outer diameter of the optical fiber. A first plug is arranged at the immersion end of or within the guiding tube proximate the immersion end of the guiding tube. The optical fiber is fed through the first plug and the first plug reduces a gap between the optical fiber and the guiding tube.

BACKGROUND OF THE INVENTION

The present invention relates to a device for measuring the temperatureof a melt, particularly of a molten metal, for example molten steel,with an optical fiber.

The Electric Arc Furnace (EAF) process for the production of moltensteel is a batch process made up of the following operations: furnacecharging of metallic components, melting, refining, de-slagging, tappingand furnace turnaround. Each batch of steel, called a heat, is removedfrom the melting furnace in a process called tapping and, hence, areference to the cyclic batch rate of steel production is commonly aunit of time termed the tap-to-tap time. A modern EAF operation aims fora tap-to-tap cycle of less than 60 minutes and is more on the order of35-40 minutes.

Many of the advances made in EAF productivity that promote rapidtap-to-tap times possible are related to increased electrical powerinput (e.g., in the range of 350-400 kWh/t), and alternative forms ofenergy input (e.g., oxygen lancing, oxy-fuel burners) into the furnace.Most advanced EAF operations consume on the order of 18-27 Nm³/t ofsupplemental oxygen which supplies 20-32% of the total power input. Inaddition, improvements to components which allow for faster furnacemovement have reduced the amount of time in which the furnace standsidle. The industrial objective of EAF operators has been to maximize thefurnace power-on time, resulting in maximum productivity in order toreduce fixed costs, while at the same time gaining the maximum benefitfrom the electrical power input. The majority of time consumed in theproduction of one heat of steel in the EAF is in the process step ofmelting.

The melting period is the heart of EAF operations and, in the majorityof modern EAFs, occurs in a two stage process. Electrical energy issupplied via graphite electrodes and is the largest contributor in themelting operation. To melt steel scrap, it takes a theoretical minimumof 300 kWh/t. To provide the molten metal with a temperature above thatof the melting point of steel requires additional energy. For typicaltap temperature requirements, the total theoretical energy requiredusually lies in the range of 350-400 kWh/t. However, EAF steelmaking isonly 55-65% energy efficient and, as a result, the total equivalentenergy input is usually in the range of 650 kWh/t for most modernoperations with 60-65% supplied by electric power, the remainingrequirements supplied by fossil fuel combustion and the chemicaloxidation energy of the refining process.

During the first metallic charge, an intermediate voltage tap is usuallyselected until the electrodes can sufficiently bore into the scrap. Theposition of unmelted scrap between the electrode arc and the side wallof the melting vessel protects the furnace structure from damage suchthat a long arc (high voltage) tap can be used after boring.Approximately 15% of the scrap is melted during the initial bore-inperiod. Fossil fuel combustion added through special nozzles in thefurnace wall contributes to scrap heating and thermal uniformity. As thefurnace atmosphere heats up, the arcing tends to stabilize and theaverage power input can be increased. The long arc maximizes thetransfer of power to the scrap and the beginnings of a liquid pool ofmetal will form in the furnace hearth. For some specific EAF types, itis a preferable practice to start the batch melting process with a smallpool held over from the previous heat called a “hot-heel”.

When enough scrap has been melted to accommodate the volume of secondcharge, the charging process is repeated. Once a molten pool of steel isgenerated in the furnace, chemical energy may now be supplied viaseveral sources, such as oxy-fuel burners and oxygen lancing. Oxygen canbe lanced directly into the bath once the molten metal height issufficient and clear of obstructive scrap.

Nearing the time that the final scrap charge is fully melted, thefurnace sidewalls can be exposed to high radiation from the arc. As aresult, the voltage must be reduced or the creation of a foamy slag thatenvelops the electrodes. The slag layer may have a thickness of morethan a meter while foaming. The arc is now buried and will protect thefurnace shell. In addition, a greater amount of energy will be retainedin the slag and is transferred to the bath resulting in greater energyefficiency. This process will create a lot of heat in the slag layercovering the steel, resulting in temperatures that are up to 200° C.higher than the steel temperature creating a very unique and difficultenvironment for process control measurements for reasons explainedlater.

Reducing the tap-to-tap time for a heat, in many instances andespecially in modern EAF operations operating with a hot heel, oxygenmay be blown into the bath throughout the heat cycle. This oxygen willreact with several components in the bath including aluminum, silicon,manganese, phosphorus, carbon and iron. All of these reactions areexothermic (i.e., they generate heat) and will supply energy to aid inthe melting of the scrap. The metallic oxides which are formed willeventually reside in the slag.

When the final scrap charge and raw materials are substantially melted,flat bath conditions are reached. At this point, a bath temperature anda chemical analysis sample will be taken to determine an approximateoxygen refining period and a calculation of the remaining power-on timeuntil tap.

Regardless of the specific local processing steps that may varydepending upon the utilization of available raw material, furnacedesign, local operating practices and the local economies of production,it is evident that many forms of energy inputs to the furnace may beemployed in a variety of strategies in order to minimize the tap-to-taptime and improve energy efficiency during the conversion of solid scrapand slag components to molten steel and slag at the correct chemicalcomposition and desired temperature for tapping.

As in other steelmaking processes, the tap-to-tap production process ofan EAF is guided by mathematical models that take into account thequantity and quality of raw materials in order to predict the processend point given the energy inputs and heat outputs. A listing of suchvariables can be found in EP 0747492 A1. Many of the process models usedto control and predict EAF performance are well known in the art. Whencompared to the classic steelmaking process of blast furnace toconverter, the variance of the raw materials used in the EAF process ismuch higher and as such require constant adjustments. One of severalinformation inputs to these models required to correct and guide theprocess is the molten metal temperature.

Providing the EAF operator with the best and most recent molten metaltemperature information should satisfy the following requirements:

an accurate temperature representative of the bulk metal,

fixed immersion depth independent of the furnace tilt,

continuously or nearly continuously available, and

bath level determination for immersion depth adjustments.

Typically, a temperature measurement of the molten metal is accomplishedusing well known disposable thermocouples such as described in U.S. Pat.No. 2,993,944. Such thermocouples can be immersed manually by anoperator with a steel pole with adapted electrical wiring andconnections to convey the thermocouple signal to appropriateinstrumentation. Additionally, many automatic thermocouple immersionmechanical systems are now utilized to provide thermocouple immersions,such as those publically available from www.more-oxy.com or described inthe literature Metzen et al., MPT International April 2000, pp. 84.

Once pooling of molten metal is established, the bath temperature willslowly increase. The higher the content of the non-molten scrap thelower the rate of temperature increase will be for a given energy input.Once all the scrap is molten, the temperature of the bath will increasevery rapidly, in the order of 35° C.-70° C./minute toward the end of theprocess. In order to predict the optimum process end, the time that themetal is ready to tap, the process control models need to havetemperature information that is accurate and at a sufficiently highfrequency of measurements to create an accurate forecast of the bestmoment to stop the various energy inputs. The measuring process usingrobotic immersion devices requires that an access hatch, typically theslag door, a general description of which appears in U.S. PatentApplication Publication No. 2011/0038391 and in U.S. Pat. No. 7,767,137,is opened to allow insertion of a mechanical arm supporting a disposablethermocouple. In most modern operations, this door is also used toprovide access to the furnace for oxy-fuel burners and oxygen lancesthat are brought into position with a similar manipulator to that of theimmersion lance. More recently, several additional ports may also beprovided around the circumference of the furnace shell for burners asdescribed in U.S. Pat. No. 6,749,661.

Opening of the slag door for the purpose of obtaining temperature latein the process allows a large amount of air to enter the furnace.Consequences of this opening are cooling the local area and providing asource for nitrogen. During arcing, nitrogen is converted to NOx whichis an undesirable effluent of the EAF process. While it is necessary todeslag the furnace through this opening, the use of robotic immersionequipment also utilizing this opening to take temperatures exposes thefurnace interior to unnecessary nitrogen ingress and unintentionalde-slagging during periods when repeated temperature measurements arerequired.

With a rapid temperature rise during the end stages of the metalrefining process, the update time for a process control model under thebest of circumstances cannot keep up with modern high powered furnaces.Ideally, rapid temperature updates during the end of refining andcontinuous temperature information during the last minutes prior to tapprovide the best combination for model prediction accuracy and end pointdetermination. A realistic test-to-test time of one minute for typicalrobotic systems limits the usefulness of spot measurements of such adynamic process. Conventional disposable thermocouples and roboticimmersion equipment suffer from several additional limitations besides alow sampling frequency that ultimately reduces the predictive success ofthe process models when used for accurate end point decisions.

During the melting and refining processes, the bath will have atemperature gradient whereas the surface of the bath will have asignificantly higher temperature than that of the bulk molten metal. Hotand cold spots of metal are located throughout the furnace interiornecessitating the use of specialized burners and directional fossilfueled heaters to help homogenize the interior. As indicated in EP1857760 A1, one cold spot is in the area of the slag door where theimmersion of disposable thermocouples typically occurs due to the largeaccess requirements of the typical robotic immersion equipment. An EAFhas the ability to “rock” furnace, that is, to tilt the horizontalposition of the furnace, front to back, in order to further homogenizethe bath, deslag and tap the furnace, as described in U.S. Pat. No.2,886,617.

Most all robotic immersion devices are mounted in the area of the slagdoor and are mounted on the operating floor, and thus do not tiltthrough the angle of the tilted furnace. Consequently, such manipulatorscannot position disposable thermocouples into the bath at all times andunder all circumstances. Furthermore, the immersion depth of athermocouple is linked to the articulation of the mechanical arm of therobotic device and, as such, cannot readily adjust to a bath levelchange due to the angle of the furnace tilt. While it is important torepeatedly measure in a location that reflects the bulk temperature forthe purpose of the operating models of the EAF process, the actualtemperature measurements taken with either a manual or automatic lanceshow difficulties towards stable immersion depths, not available whilethe position of the immersion lance is not aligned to the rocking of thefurnace and the actual bath level, and not in a location conducive totemperature accuracy.

There are numerous temperature measuring devices in the prior artinstalled in a variety of steelmaking vessels that utilize permanentoptical light guides to focus the radiation toward the opticaldetectors. Examples of such prior art devices can be found in JP-A61-91529, JP-A-62-52423, U.S. Pat. No. 4,468,771, U.S. Pat. No.5,064,295, U.S. Pat. No. 6,172,367, U.S. Pat. No. 6,923,573, WO 98/46971A1 and WO 02/48661 A1. The commonality of this prior art is that theoptical guides are permanent and, as a result, need to be protected fromdamage using complicated installations. These protective means maycomprise gas purging to either cool the assembly or remove the metalfrom physical contact with the optical element, layers of protectivesheathing that are relatively permanent or slightly erodible with thelining of the steelmaking vessel and complicated emissivity correctionof the light wavelength(s) and intensity in order to determine anaccurate temperature.

JP-A-08-15040 describes a method that feeds a consumable optical fiberinto liquid metal. The consumable optical fiber, such as disclosed inJP-A-62-19727, when immersed into a molten metal at a predictable depthreceives the radiation light emitted from the molten metal at blackbodyconditions, such that the intensity of the radiation using aphoto-electric conversion element mounted on the opposite end of theimmersed consumable optical fiber can be used to determine thetemperature of the molten metal. The scientific principle of the priorart concisely detailed in P. Clymans, “Applications of an immersion-typeoptical fiber pyrometer”, is that the optical fiber must be immersed ata depth to achieve blackbody conditions. Continuous measurements ofmolten metals using consumable optical fiber and equipment necessary tofeed long lengths of coiled material to a predetermined depth are wellknown in the art, such as EP 0806640 A2 and JP-B-3267122. In harshindustrial environments where the consumable optical fiber is immersedinto higher temperature metals or in the presence of metals with a slagcovering maintaining a predetermined depth during the period of time themeasurement should take place has proven to be difficult due to theinherent weakness in the optical fiber as its temperature increases. Ithas become necessary to protect the already metal covered fiber withadditional protection such as gas cooling as disclosed inJP-A-2000-186961, additional composite materials layered over the metalcovered fiber as disclosed in EP 655613 A1, insulating covering asdisclosed in JP-A-06-058816, or additional metal covers as disclosed inU.S. Pat. No. 5,163,321 and JP-B-3351120.

The above improvements for high temperature use have the disadvantage ofdramatically increasing the cost of the consumable fiber assembly inorder to provide a continuous temperature reading. Although not exactlyidentical to the conditions encountered when measuring highertemperatures in an EAF, JP-B-3351120 is useful to have an appreciationof the speed of consumption of the optical fiber. In the disclosedexample using a very complex mechanical device for feeding, an opticalfiber from a coil is used. The coil consists of the metal coveredoptical fiber covered again with additional 3 mm thick stainless steeltubing. The disclosed calculations recommended for improved temperatureaccuracy for continuous temperature measurements in iron of a blastfurnace tap stream is an astonishing 500 mm/s. The cost of the opticalfiber and its enclosing stainless steel outer tube are costly to consumeat this recommended feeding rate.

A practical economy of continuous temperature measurements depends uponconsuming the least amount of fiber possible while still obtaining thebenefit of continuous information. Bringing the optical fiber to themeasuring point with the least amount of exposed fiber is described inU.S. Pat. No. 5,585,914 and JP-A-2000-186961, where a single metalcovered fiber is fed through a permanent nozzle that could be mounted inthe furnace wall and through which gas is injected. While these devicescan successfully deliver the fiber to the measuring point, they become aliability due to clogging and continuing maintenance. At stages in thefeeding mode, vibration is required to prevent the fiber from welding tothe nozzle. If the port is blocked or closes due to inadequate gaspressure, the measurement is terminated with no possibility ofrecovering until the nozzle is repaired. EP 0802401 A1 addresses theproblem of a blocked opening to the furnace with a series of punch rodsand guide tubes positioned on a movable carriage, providing a tool setfor addressing whichever problem prevents the fiber from passing throughthe nozzle. However, these are strategies to unblock a closed accessport from which no measuring data can be obtained. Once these ports areblocked there is no possibly to obtain temperature data, which could beat critical times in the steelmaking process.

An additional problem arises for continuously fed optical fibers thatfurther increase the cost of measurement and the complexity of theimmersion equipment. The immersion type optical fiber only maintains itsoptical quality, and thus returns and accurate temperature if it staysprotected against heat and contamination or is renewed at a rate higherthan its degradation rate. The optical signal of the bath temperature isaccurately obtained in blackbody conditions for the part immersed in themolten steel. However, the remaining un-immersed portion part above mustremain a perfect light guide. At elevated temperatures, devitrificationof the optical fiber will occur, the transmissivity of the lightdecreases and an error in temperature as a function of decreasedintensity increases. JP-A-09-304185 and U.S. Pat. No. 7,891,867 disclosea feeding rate method where the speed of fiber consumption must begreater than the rate of devitrification, thereby assuring that a freshoptical fiber surface is always available. Simple laboratory testingshows that the optical signal stays stable during a very short period,being around 1.0 s at temperatures below 1580° C. and only 0.1 s whileimmersed at 1700° C. Although a solution for lower temperature metals,the speed of feeding optical fiber at a speed greater than thedevitrification rate for elevated temperature testing is expensive for asimple metal covered optical, fiber. In the case of measuring elevatedtemperatures in the harsh conditions of an EAF, the prior art disclosedextra protection methods are also consumed at the same rate of as theoptical fiber. This becomes prohibitedly expensive for the abovementioned double covered optical fibers.

JP-A-2010-071666 discloses a fiber optical temperature measuring devicefor measurements in molten metal using an airtight environment and ameasuring lance having an airtight sealing between lance tube andoptical fiber.

BRIEF SUMMARY OF THE INVENTION

The present invention measures temperature in a metallurgical vesselusing a molten metal immersed consumable optical fiber and immersionequipment capable of inserting a temperature device through the sidewall of an EAF to a predictable molten steel immersion depth with atemperature-to-temperature measuring frequency of less than 20 seconds.The ability to sample on-demand, singularly or, in rapid successionallows a measuring strategy that can update a mathematical predictivemodel for EAF operations at key times during the process with theability to measure in rapid succession providing near continuoustemperature data at a low cost.

The present invention works away from the prior teaching preferring toprovide a spot measurement rather than a continuous measurement. In oneembodiment, the present invention is a low cost solution for temperaturemeasurements suitable to be utilized at a sufficiently high samplingfrequency to meet the updating demands of the mathematical models of theEAF melting process while solving the problems associated with immersedoptical fiber in harsh environments. The present invention provides anear continuous temperature measuring output comprised of immersing anoptical fiber into the molten metal through the slag covering withoutfirst contacting the slag, maintaining a predetermined immersion depthduring the measuring period by controlled feeding, protecting thenon-immersed portion against devitrification in the high ambient heat ofthe EAF interior, removing and recoiling unused fiber after themeasurement, measuring the bath level upon recoiling and an immersionequipment for repeating the measuring processes always duplicating theinitial starting conditions.

One of the problems solved by the present invention is to improve theknown methods and devices. Providing the EAF operator with the best andmost recent molten metal temperature information should satisfy at leastthe following requirements:

an accurate temperature representative of the bulk metal,

fixed immersion depth independent of the furnace tilt,

continuously or nearly continuously available, and

bath level determination for immersion depth adjustments.

In one embodiment, the present invention is directed to a method formeasuring the temperature of a melt, particularly of a molten metal,with an optical fiber, wherein the optical fiber is fed into the meltthrough a disposable guiding tube and whereby the optical fiber and animmersion end of the disposable guiding tube are immersed into the meltboth having a feeding speed whereby both feeding speeds are independentfrom each other. Preferably, in a first phase of immersion thedisposable guiding tube and the optical fiber are immersed into the meltand, in a second phase, the optical fiber is immersed with higher speedand deeper into the melt than the disposable guiding tube. It ispreferred that the second phase starts after the immersion end of thedisposable guiding tube is immersed into the melt. Further, it ispreferred that in a third phase of immersion the optical fiber isstopped or is withdrawn from the melt.

In a preferred embodiment of the method, the speed of the disposableguiding tube and/or of the optical fiber is varied during immersion.Further, it is preferred that the optical fiber and the disposableguiding tube are moved with unequal speed. It is advantageous that, inaddition to the temperature, also the upper surface of the melt isdetermined.

In another embodiment, the present invention is directed to a device formeasuring the temperature of a melt, particularly of a molten metal,comprising an optical fiber and a (preferably disposable) guiding tube,having an immersion end and a second end, opposite to the immersion end,is characterized in that the optical fiber is partially arranged in thedisposable guiding tube, whereby the inner diameter of the guiding tubeis bigger than the outer diameter of the optical fiber, whereby a firstplug or a reduction of the diameter of the guiding tube is arranged atthe immersion end of or within the guiding tube approximate theimmersion end of the tube and whereby a second plug may be arranged atthe second end of or within the guiding tube approximate the second endof the tube, whereby the optical fiber is fed through the plugs or thereduction of the diameter of the guiding tube and whereby the first andpreferably also the second plug or the reduction of the diameter of theguiding tube reduces or even closes a gap between the optical fiber andthe guiding tube. The reduction of the diameter of the guiding tube canalternatively also be understood as a reduction of the cross-sectionarea of the tube at or approximate its immersion end. The guiding tubecan preferably be disposable, that means, that it can easily be replaced(for example, if damaged) without necessarily using tools. Preferablythe area of the gap is reduced to less than 2 mm², more preferably lessthan 1 mm². It can even be closed. Preferably, one or both of the plugsare elastic, more preferably made of elastic material. It is furtherpreferred that the distance of the immersion end of the first plug orthe reduction of the diameter of the guiding tube from the immersion endof the guiding tube (if the first plug is arranged in the guiding tube)is not more than 5 times the inner diameter of the guiding tube. If thesecond plug is arranged within the guiding tube, it is preferablyarranged between the first plug or the reduction of the diameter of theguiding tube and the second end of the guiding tube.

Preferably, at least the first plug (or the first and the second plug)has a conical shape, at least at its immersion end, whereby the wallthickness of the plug is reduced towards the immersion end. It may beadvantageous that the inner diameter of at least the first plug isreduced towards the immersion end.

It is preferred that the device further comprises a fiber coil and afeeding mechanism for feeding the optical fiber and the guiding tube,whereby the feeding mechanism comprises at least two independent feedingmotors, one for feeding the optical fiber and one for feeding theguiding tube. Preferably, the device is characterized in that thefeeding motors are each combined with a separate speed control.

Further, another embodiment of the present invention is related to amethod of use of such a device, as described above in a method asdefined by the foregoing description.

In one embodiment, the present invention is utilized to obtaintemperature measurements needed to control the final processing steps ofsteelmaking in an EAF. To be useful for this purpose, the device of thepresent invention preferably provides:

-   -   accurate temperature measurements at a sampling frequency that        provides accurate updating of the process model and operator        information towards tapping,    -   intermediate measuring which provides the lowest cost, and    -   a metal measurement position representative of the metal        temperature.

The device of the present invention preferably comprises a continuoustemperature measuring element and a fiber which is preferably alwaysconnected to the instrumentation, such that the device is alwaysavailable, there is no loss of availability waiting for connections,there is a rapid response time and low contact time in metal and slag,and there is low cost. The device preferably further comprises an outermetal tube which supports the fiber during rapid acceleration towardsthe bath so as to avoid bending away from metal, guarantees that thefiber enters the metal so as to avoid deflection upwards towards theslag, keeps the fiber from contacting liquid slag so as to avoidcontamination, and keeps the non-immersed portion of the fiber cool soas to avoid devitrification. The outer metal tube further is preferablya guide that retains straightness of the withdrawing optical fiber so asto prepare the fiber for the next usage, is disposable (anew straightpiece is used each time so there are guaranteed dimensions) and isflexible to accommodate a non-ideal fiber end. Preferably, the devicefurther includes gas plugs that enclose a volume of gas within the tubeso as to allow for the creation of a positive pressure within the tube.

In one embodiment, the present invention involves immersing the opticalfiber in the steel bath over a sufficiently long length using a machinethat:

-   -   is mounted on the EAF side wall;    -   has a preferred 20 s cycle time,    -   monitors the location of the end of the fiber at all times by        directly and indirectly using encoders and inductive position        devices,    -   refurbishes the outer tube and gas plugs and positions the fiber        inside and through both,    -   ejects a used outer tube and gas plugs into the EAF while        recoiling unused fiber,    -   is capable of +2000 mm/s feed with near instant deceleration,    -   inserts fiber and outer tube into the EAF at differential        velocities,    -   has reversible and independent reversible drive capabilities        (moving in opposite directions),    -   has momentum compensating actuators for de-spooling and        recoiling of fiber, and    -   has remote instrumentation for temperature and bath level        detection.

U.S. Pat. No. 5,585,914 recognizes that intermittent optical fiberfeeding provides intermittent temperatures. When the on-demandtemperature availability is sufficient to guide the metallurgicalprocess, then the requirement for continuous temperature becomesunsupported by the technical need for such data.

In the above disclosure, a 10 mm/s feed for 10 s with a 20 s off timewas described to be adequate for the LD process. During the off time,the fiber must be vibrated in order to prevent the outer jacket fromwelding to the nozzle. During both the feeding and waiting times gas ispurged through the nozzle whose diameter is fixed by the OD of the outerfiber jacket to be between 1.8 mm and 4.2 mm. Through this nozzle flowsa purged gas contained by a series of rubber plugs contained in ahousing supplied with oil.

EP 0802401 A1 also provides for on-demand temperature readings of a 2-3s duration utilizing an optical fiber fed through a gas purged guidetube or an “extension means” for the purpose of protecting the extended(but not immersed), portion of the optical fiber. Both of these outertubes are not consumable. An immersion machine is equipped to cut offthe devitrified portion of the optical fiber so a fresh surface ispresented every 4-5 immersions.

JP-B-3351120 discloses a continuously fed metal covered optical fiberwith an additional consumable outer metal tube, both of which are fedinto the metal at the same time. A feeding machine is also described.The consumable protective tube of JP-B-3351120 was continuously presenton the outside of the fiber as if it were an integral part of the fiber.The present invention, however, utilizes a disposable outer tubeseparate and distinct from the optical fiber. One could not feed theouter metal tube of JP-B-3351120 without also feeding the fiber. Theseparation of an additional outer metal tube from the optical fiber is abenefit of the present invention. The present invention also providessolutions to other problems. While EP 0802401 A1 recognizes the need foran extension or guide tube to aid the immersion of the fiber, the guidetube does not extend completely to the metal surface. It is notimmersible and not disposable and, because of this, the optical fiber isnever completely secure.

In practice, the extension or guide tube may be treated as a nozzle, andboth suffer from problems of blockage. In fact, both the describednozzle and the guide tubes have additional mechanisms to avoid blockageof their apertures from material ingress. The prior art clearlyrecognizes the importance of a purge gas to prevent slag/steel formentering a nozzle through which the fiber is fed. Since these nozzlesare not disposable, the method for sealing the purge gas between theguide tube and the immersion end are typical permanent seals with oil.

According to the present invention, the disposable outer tube with atleast one (preferably disposable) gas plug provides a well containedsystem. The system of the present invention can use the thermalexpansion of the gas already present in the tube, behind the first plug,or preferably between the two plugs, instead of adding an external purgegas thereby solving problems of purge gas supply inherent in the priorart. In EP 0802401 A1, the guide or extension tube does not contact themetal. Its open end cannot provide for pressurization during heated gasexpansion. In the permanent enclosed space of U.S. Pat. No. 5,585,914,once the gas has expanded, it can no longer provide a displacement forthe metal ingress. In JP-B-3351120, the space between the outer tube andfiber is finitely long and, due to the compressibility of gas, cannot beused to provide a heated expansion of gas at the immersion end. Theuniqueness of a self-purging outer tube can only be possible with theconception of disposability of the outer tube. This feature of oneembodiment of the present invention is unique among the entire priorart. Further, this feature is not obvious because the prior art wassolving problems related to maintaining a continuous measurement from acontinuously fed optical fiber.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

In the following the invention is described by way of an example.

FIG. 1 shows a prior art consumable optical fiber;

FIG. 2A shows the leading section of a metal coated optical fiber withguiding tube;

FIG. 2B shows an alternate leading section of a metal coated opticalfiber with guiding tube;

FIG. 2C shows a further alternate leading section of a metal coatedoptical fiber;

FIG. 3 shows the leading section of a metal coated optical fiber withadapted guiding tube;

FIG. 4A shows an immersion device before immersing the optical fiber;

FIG. 4B shows the immersion device after immersing the optical fiber;

FIG. 4C shows the immersion device according to FIG. 3B with a differentmelt container such as a molten metal ladle or tundish; and

FIG. 5 shows a view of both the position of the immersion end of theouter tube and the immersion end of the optical fiber during immersion.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the device of the present invention is described asfollows, by way of example. FIG. 1. shows a prior art consumable opticalfiber 10, typically employed in the measurement of liquid metalscomprising an optical fiber, a jacket covering the optical fiber and aprotective metal tube covering the surface of the plastic jacket. Theoptical fiber 10, typically a graded index multimode fiber made ofquartz glass with an inner core 11, diameter of 62.5 μm and an outercladding 12, diameter of 125 μm covered with a polyimide or similarmaterial 13. The protective metal tube 14 is typically stainless steel1.32 mm outer diameter (OD) and 0.127 mm wall thickness. Although ametal covered optical fiber is preferred, additional embodiments whereprotective metal tube 14 and/or polyimide or similar material 13 arereplaced by a singular plastic material do not depart from the intendedinvention.

FIG. 2A shows the leading section 10′ of a metal coated optical fiber10, as fed from spool 20 through a first gas retaining elastic plug 32,affixed to an outer disposable guiding tube 40. The first gas retainingplug 32 is proximate the immersion end 50 of tube 40.

FIG. 2B shows the leading section 10′ of a metal coated optical fiber10, as fed from spool 20 through a second gas retaining elastic plug 30,affixed to the opposite immersion end 50 of an outer disposable guidingtube 40. A first gas retaining plug 32 is proximate the immersion end 50of tube 40.

Fiber 10 and outer disposable guiding tube 40 are not in a fixedarrangement and, as such, can move independent of each other and thuscan be independently inserted through the slag layer 51 and into themolten bath 52 at different velocities while maintaining a gas volume 31between plugs 30 and 32. Disposable guiding tube 40 is preferably lowcarbon steel having a wall thickness of 0.8 to 1 mm, but may be selectedfrom a variety of metal materials as well as ceramics and glasses,cardboard and plastics or a combination of materials. In the case thatdisposable guiding tube 40 is selected from a material that reacts withthe molten bath, the immersion portion 50 is preferably prepared in away that it does not splash molten metal on the inside of the disposableguiding tube 40 by the application of coating or coverings of a materialknown in the art for the purpose of splash reduction.

Immersing the open ended outer disposable guiding tube 40 in the steelthrough the slag layer 51 without plug 30 will result in ingress of slagand steel in this tube. Molten slag resulting from the refining processis preferably high in oxides, such as iron oxide which is easilyabsorbed into the optical fiber structure. The fiber 10 fed through theouter disposable guiding tube 40 containing slag and steel will bedamaged before reaching the open end of the outer disposable guidingtube 40. For the preferred outer disposable guiding tube 40, of 2 m longwith an immersion depth of 30 cm and being open at both ends, theupwelling of molten material inside the outer disposable guiding tube 40can be up to 30 cm. In case of a closed end outer disposable guidingtube 40, the upwelling will be approximately 16 cm. This is calculatedignoring the gas expansion of the enclosed air which will undergoexpansion due to an increase in its temperature. Tests show that thesteel ingress can be minimized by reducing the air gap between the innerdiameter (ID) of the outer disposable guiding tube 40 and the OD of theoptical fiber 10 metal covering. Preferably, the air gap is reduced tothe minimum. However, practically for tubes with an ID of 10 mm, thisgap is preferably less than 2 mm², more preferably less than 1 mm². Itcan even be closed. Tubes with a smaller ID would allow a bigger gap dueto the faster heating rate of the enclosed air.

One of the preferred features of the present invention is to avoidmolten ingress utilizing the thermal expansion of a volume of gascontained between a pair of gas retaining plugs affixed on or within inthe disposable guiding tube 40. The use of elastic plugs 30 and 32 toeffectively seal the end opposite the immersion end of a certain sealingquality ensures that expanding gas retains a positive pressure againstthe filling pressure of the liquid steel during immersion, thus keepingthe disposable guiding tube 40 clear. Notwithstanding, any means ofcreating an overpressure in the disposable guiding tube 40 whileimmersing also avoids steel ingress such as an internal coating of amaterial vaporous at minimal temperatures, such as a galvanized coating(for example Zn). A prominent concept towards creating a positivepressure in the outer disposable guiding tube 40 is to avoid theupwelling and intrusion of metal, slag or other contaminants inside thedisposable guiding 40 tube that could impede the free feeding of theoptical fiber 10.

Plugs 30 and 32 preferably have a feed through hole having a diameter(non-operated) which his less than the outer diameter of the opticalfiber and should be suitably elastic in order to compensate for anun-ideal optical fiber end resulting from the prior immersion. Thethermo elastic material Santoprene® (Santoprene is a trademark of ExxonMobile) is one such material that has been found to both remain elasticand surprisingly intact during the duration of the measurement. However,it can also be a different material, such as wood or another suitableplastics. In one embodiment, plugs 30 and 32 are preferably replacedwith each outer disposable guiding tube 40. Each replacement assures aproper seal, however plug 30 could be constructed in such a way as to bereused with multiple outer disposable guiding tubes and replaced as amatter of maintenance. The preferred location of the plug 30 at theterminal end of outer disposable guiding tube 40 in FIGS. 2B and 2C isselected for ease of installation. However, placing plug 30 closer tothe immersion end is equally acceptable. The design of plugs 30 and 32in FIG. 2B facilitates their placement at the extremities of disposableguiding tube 40 showing lips that rest upon the tube ends. Otherconfigurations are possible as well as means molded or embossed on theexternal surface of the plug to aid in the fixation of the plugs to tube40 by tabs or adhesives. The exact embodiment of plug 32 should reflectthe ease of positioning, location and fixation of its position withoutdeparting for the main purpose of the plug to restrict the escape of airfrom the outer tube, thus ensuring a build-up of inner pressure. FIG. 2Cshows an alternate position for gas retaining plug 32 approximate theimmersion end of tube 40. In this embodiment, the preferred distancebetween the immersion end of tube 40 and the exit location of theoptical fiber from plug 32 is not more than 5 times the internaldiameter of tube 40. Opposite the immersion end of plug 32, the internalcontour of the plug tapers towards the internal wall of tube 40 suchthat the thickness of the plug 32 at its extremity is not more than onethird the diameter of the optical fiber, thus insuring a consistentguide towards the immersion end during feeding. Plug 32 also may beconfigured with means molded or embossed on the external surface of theplug to aid in the fixation of the plugs to tube 40 by tabs oradhesives. The plugs have a conical shape, whereby the wall thickness ofthe plugs is reduced towards the immersion end.

Similar to FIG. 2C, a reduction of the diameter or of the cross-sectionarea of the guiding tube 40 at or close to its immersion end can be used(FIG. 3) instead of plug 32.

The steel ingress in the steel tube while immersing in the steel tubepreferably increases with an increase of the immersion depth, anincrease of the tube length, an increase of the air gap (at the otherend), a lower bath temperature, a thicker wall thickness and a higheroxygen content of the steel bath.

The immersion device is shown in FIGS. 4A-4C. Machine 100 is suitablyconstructed and instrumented in such a manner that assembly of plugs 30and 32 to outer disposable guiding tube 40 are aligned so optical fiber10 can be inserted through plug 30 into the interior of the outerdisposable guiding tube 40 and just exit plug 32. Notwithstanding, tube40 and plugs 30 and 32 can be preassembled and loaded on machine 100without departing from the scope of the invention. Both the outerdisposable guiding tube 40 and optical fiber 10 are fed at approximately3,000 mm/s through the side wall of an EAF through suitable accesspanels 80. These panels 80 are not part of the machine 100. The machine100 has independent 100% reversible drive or feeding motors 25; 45.Motor 25 drives the optical fiber 10 and motor 45 drives the disposableguiding tube 40, so that the velocity of the outer disposable guidingtube 40 in either direction is independent of the velocity of theoptical fiber 10 in either direction.

The machine 100 is preferably capable of independent feeding of opticalfiber 10 into the bath with a speed less, equal or higher than the speedof the outer disposable guiding tube 40. Preferably, the optical fiber10 is fed faster so that both the immersion end 50 of the outerdisposable guiding tube 40 and leading section 10′ of optical fiber 10arrive at the predetermined surface of the metal at approximately thesame time. Once the bath level position is reached, the outer disposableguiding tube 40 is decelerated to a nearly stationary position in themolten metal 52. The leading section 10′ of optical fiber 10 continuesto move slowly deeper in the steel at about 200 mm/s for approximately0.7 s. Both the outer disposable guiding tube 40 and the optical fiber10 are constantly moving at unequal speeds to avoid welding the twometal surfaces together, thereby solving a problem stated in the priorart.

The problem of the acceleration and deceleration of the optical fiber 10is more complicated than moving the outer disposable guiding tube 40.The optical fiber 10 is constantly de-coiled and recoiled from a coil orspool 20 with its coil weight that is constantly changing due to fiberconsumption. The feeding machine must be adapted with additionalmechanics to avoid the elastic spring back effect from the coil or spool20 itself, as well as the weight of the pyrometer connected to the coil.This is solved by using a servo motor or feeding motor 25 to control thefiber movement. One feeding motor 25 takes care of the de-coiling andrecoiling of the fiber 10 and pre-feeds fiber 10 in such a way that thefeeding motor 25 can accelerate very fast.

The consumable optical fiber 10 receives the radiation light emittedfrom the molten metal, conveys such to a photo-electric conversionelement mounted on the opposite end of the coiled consumable opticalfiber and combined with associated instrumentation measures theintensity of the radiation, using this to determine, the temperature ofthe metal. The optical fiber coil or spool 20 and instrumentation arelocated at a distance away, and separated from the EAF but are suitablyrobust to withstand the harsh conditions of the steel makingenvironment. The location of the immersion end of the optical fiber 10is preferably constantly known and monitored by machine instrumentationthroughout the immersion, measuring and removal portions of theimmersion cycle. The machine is preferably equipped with positionencoders that determine the passage of fiber length and inductiveswitches that register the fiber end.

After the measurement is complete, both the consumable optical fiber 10and the outer disposable guiding metal tube 40 are withdrawn from thesteel with different speeds in such a way that the optical fiber 10stays relatively deeper in the bath. During this movement, it is capableto determine the bath-level due to a change in the light intensity whencorrelated with the length of optical fiber 10 extracted betweenpredetermined positions. The post measurement bath level determinationis subsequently used for the next immersion. It is also contemplatedthat the bath level could be determined during immersion using varioustechniques well described in the literature without departing from themethod of the present invention.

Once the optical fiber 10 is clear of the EAF interior, the direction ofthe outer disposable guiding tube 40 is preferably reversed toward thefurnace interior. The outer disposable guiding metal tube 40 is thenejected, disposed and consumed in the furnace interior. A new outerdisposable guiding tube 40 and gas plugs 30 and 32 are positioned toreceive the optical fiber 10 for the next measurement. The remainingoptical fiber 10 is preferably recoiled during removal and returned to astarting position.

Key abilities of embodiments of the present invention are:

accurate payout and recoil of fiber,

detection of fiber end,

loading of outer disposable guiding tube,

load and position of gas plugs,

guide fiber at starting position into gas plugs,

fully reversible drives for both fiber and outer disposable guidingtube,

independent speed profiles for fiber and outer disposable guiding tube,

registration of fiber output for level detection, and

attachable to furnace shell for tilt compensation of bath level.

The method of one embodiment of the present invention is described byway of example of a total cycle description. This concept preferablyinvolves an operator free control of EAF's. It is envisioned that thebest operation is to take multiple temperature immersions in quicksuccession (about 5). Each immersion is preferably approximately 2 s;the total cycle time is preferably less than 20 s during a single heat.

The schematic of FIG. 5 gives a view of both the position of theimmersion end 50 of the outer disposable guiding tube 40 and theimmersion end or leading section 10′ of the optical fiber 10 during 2immersions of a measurement cycle. For fiber movement, the end positionof the fiber is preferably tracked.

With tube movement is indicated the position of the immersed end of thedisposable guiding tube 40. At or near the immersion end of tube 40 isgas plug 32. At the opposite of the immersion end 50 of the outerdisposable guiding tube 40 is the gas plug 30. For the purpose of thisschematic, the outer disposable guiding tube 40 is already in ready toimmerse position. Gas plugs 30 and 32 are already attached to the backend and the optical fiber 10 is slightly extended from the gas plug 32towards the molten metal. The relative dimensions shown are fordescriptive purposes understanding that the absolute distances arepredicated upon the actual furnace size which is a variable from steelshop to steel shop.

The starting position 1 at time 0 of the fiber within the outer metaltube set at 350 cm above molten metal/bath-level. The starting position1 at time 0 of the immersion end of the outer metal tube is located at150 cm above the bath-level. The optical fiber 10 is fed from position 1to 2 while the outer disposable guiding tube 40 remains nearlystationary. Between time 0.8 s and 1.2 s covering positions 2 through 4both optical fiber 10 and outer disposable guiding tube 40 advance to alocation just above the molten slag 51. At 1.2 s and position 4, thefiber is advanced slightly faster than the outer disposable guidingmetal tube 40 passing through the slag 51 and into the molten metal 52.The outer disposable guiding metal tube 40 slows while the optical fiber10 advances at approximately 200 mm/s reaching the maximum immersion atposition 6 and 1.5 s into the immersion. Both optical fiber 10 and outerdisposable guiding tube 40 are extracted within 0.1 s. The optical fiber10 continues to be withdrawn and recoiled returning to its load position8 while the remains of the outer disposable guiding metal tube 40direction is reversed at position 7 and discarded. The optical fiber 10is still protected by the remaining portion of the discarded outerdisposable guiding tube 40.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

We claim: 1-11. (canceled)
 12. A device for measuring the temperature ofa melt comprising: a guiding tube having an immersion end and a secondend opposite to the immersion end, and an optical fiber partiallyarranged in the guiding tube, an inner diameter of the guiding tubebeing larger than an outer diameter of the optical fiber, wherein afirst plug or a reduction of the inner diameter of the guiding tube isarranged at the immersion end of the guiding tube or within the guidingtube proximate the immersion end of the guiding tube, and wherein theoptical fiber is fed through the first plug and the first plug reduces agap between the optical fiber and the guiding tube.
 13. The deviceaccording to claim 12, wherein a second plug is arranged at the secondend of the guiding tube or within the guiding tube proximate the secondend of the guiding tube, wherein the optical fiber is fed through thefirst and second plugs or the reduction of the inner diameter of theguiding tube, and wherein the first and second plugs or the reduction ofthe inner diameter of the guiding tube reduce a gap between the opticalfiber and the guiding tube.
 14. The device according to claim 12,wherein the area of the gap is reduced to less than 2 mm².
 15. Thedevice according to claim 12, wherein the first plug is elastic.
 16. Thedevice according to claim 15, wherein the first plug is made of anelastic material.
 17. The device according to claim 12, wherein adistance of an immersion end of the first plug or the reduction of theinner diameter of the guiding tube from the immersion end of the guidingtube is not more than 5 times the inner diameter of the guiding tube.18. The device according to claim 13, wherein the second plug isarranged within the guiding tube between the first plug or the reductionof the inner diameter of the guiding tube and the second end of theguiding tube.
 19. The device according to claim 12, wherein at least thefirst plug has a conical shape at least at its immersion end, andwherein a wall thickness of the first plug is reduced toward theimmersion end.
 20. The device according to claim 12, wherein an innerdiameter of at least the first plug is reduced toward the immersion end.21. The device according to claim 12, further comprising a fiber coiland a feeding mechanism for feeding the optical fiber and the guidingtube, the feeding mechanism comprising at least two independent feedingmotors, one for feeding the optical fiber and one for feeding theguiding tube.
 22. The device according to claim 21, wherein the twoindependent feeding motors are each combined with a separate speedcontrol.